The souring of milk is a chemical change. It involves the breakdown of lactose, a sugar in milk, into lactic acid through the action of bacteria. This chemical reaction changes the composition and properties of the milk, resulting in the sour taste and curdling. 1
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Key Takeaways: Is Milk Souring a Physical or Chemical Change?
- Milk souring is a chemical change because it involves a chemical reaction that alters the composition and properties of milk.
- The breakdown of lactose into lactic acid by bacteria during fermentation is the primary cause of milk souring.
- Milk souring is not a physical change because it is irreversible and involves the formation of a new compound (lactic acid) from lactose, unlike physical changes that retain the original composition of a substance.
Why is milk souring a chemical change?
Milk souring is considered a chemical change because it involves a chemical reaction that alters the composition of the milk. 2 When milk sours, it undergoes a process known as lactose fermentation. 3
The primary cause of milk souring is the activity of bacteria, specifically lactic acid bacteria, that naturally occur in milk. These bacteria consume the lactose, a sugar present in milk, and convert it into lactic acid through fermentation. 4
This fermentation process is a chemical reaction where the lactose molecule is broken down and transformed into lactic acid.
The production of lactic acid changes the properties of the milk. It lowers the pH level, making the milk more acidic, and gives it a sour taste and odor. 5
Additionally, the lactic acid curdles the milk proteins, causing the milk to thicken and develop a lumpy texture. 6 7
Since the lactose fermentation process involves a chemical reaction that results in the formation of a new compound (lactic acid) and changes in the physical properties of the milk, it is considered a chemical change rather than a simple physical change.
Why is milk souring not a physical change?
Milk souring is not a physical change because it involves a chemical reaction that fundamentally alters the composition and properties of the milk. Physical changes, on the other hand, are reversible processes that do not result in the formation of new substances. 8
When milk sours, the lactose present in the milk undergoes fermentation by bacteria, resulting in the formation of lactic acid. This chemical reaction changes the taste, odor, and texture of the milk.
Unlike physical changes such as melting or freezing, where the substance retains its chemical composition, milk souring involves the conversion of lactose into a different compound, lactic acid.
This transformation is irreversible, and the resulting sour milk cannot be restored to its original state simply by reversing the process. Therefore, milk souring is classified as a chemical change rather than a physical change.
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- Sankhla, M. S., Verma, R. K., Kesarwani, S., Sonone, S. S., Parihar, K., & Kumar, R. (2022). Chemistry and material studies in fermented dairy products. Advances in Dairy Microbial Products, 177–189. https://doi.org/10.1016/b978-0-323-85793-2.00017-5
- Sciencedirect.com https://www.sciencedirect.com/science/article/pii/S0021925818875433/pdf?md5=9b5c6d3c47e25f36ba5fcd6bc1833d23&pid=1-s2.0-S0021925818875433-main.pdf
- Lactic acid fermentation – Wikipedia. (2007, June 10). Lactic Acid Fermentation – Wikipedia. https://en.wikipedia.org/wiki/Lactic_acid_fermentation
- Lactic Acid Fermentations – Applications of Biotechnology to Fermented Foods – NCBI Bookshelf. (1992, January 1). Lactic Acid Fermentations – Applications of Biotechnology to Fermented Foods – NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK234703/
- Cheesy Science – American Chemical Society. (n.d.). American Chemical Society. https://www.acs.org/education/resources/highschool/chemmatters/past-issues/2017-2018/december2017/cheesy-science.html
- Lohner, S. B. (n.d.). A Milk-Curdling Activity. Scientific American. https://www.scientificamerican.com/article/a-milk-curdling-activity/
- Ever wondered how milk becomes cheese? (2015, May 15). Washington Post. https://www.washingtonpost.com/lifestyle/kidspost/ever-wondered-how-milk-becomes-cheese/2015/05/15/32abad22-ece8-11e4-8abc-d6aa3bad79dd_story.html
- Changes in Matter: Physical vs. Chemical Changes. (n.d.). Changes in Matter: Physical Vs. Chemical Changes. https://education.nationalgeographic.org/resource/changes-matter-physical-vs-chemical-changes